Emerson (enhancement) Effect
The Emerson effect is about driving photosynthesis with part of the light PAR (400-680nm in this case), and part of the light far red color (700nm-740nm or so wavelength), which combined can result in photosynthesis rates higher than normal.
Several recent studies have shown that far-red photons synergistically interact with shorter wavelength photons to increase leaf photochemical efficiency. (source)
Robert Emerson used his work with red and far red light to deduce that there must be two photosystems, called photosystem I (PSI) and photosystem II (PSII). These are named in the order of discovery, but for photosynthesis the process starts with the PSII first. Monochromatic light has a sharp drop off in photosynthesis at 680nm or so (red drop effect), but this does not happen if far red light is added with about 720nm being most efficient in driving additional photosynthesis.
Adding far-red photons (up to 40%) to a background of shorter wavelength photons caused an increase in canopy photosynthesis equal to adding 400-700nm photons. Far-red alone minimally increased photosynthesis. This indicates that far-red photons are equally efficient at driving canopy photosynthesis when acting synergistically with traditionally defined photosynthetic photons.These results suggest that farred photons (701-750 nm) should be included in the definition of photosynthetically active radiation. Shuyang Zhen and Bruce Bugbee, Far-red photons have equivalent efficiency to traditional photosynthetic photons.
- Far red light can drive the PSI independently of the PSII, and PAR is more efficient with the PSII while not as well excited with the PSI. Basically how the Emerson effect works is freeing up electrons between the PSI and PSII by driving them more efficiently in parallel, and photosynthesis becomes more efficient as a result.
- You can see this jamming of electrons in chlorophyll fluorescence shots with proteins associated with the PSII and much less fluorescence associated with the PSI (the single 750nm hump). Higher fluorescence means lower photosynthesis efficiency.
- I think most far red driver boards are gimmicks because they are likely not putting out enough far red light to make a noticeable difference.